A technique known as squeeze or remedial cementing is a common operation in the petroleum industry. Most squeezes are performed with a drilling or workover rig and through threaded tubing or drillpipe. Squeeze cementing is most often performed to repair leaks in well tubulars and restore pressure integrity to the wellbore, raise the level of or restore a cement sheath behind the casing to support or protect well tubulars, modify the production or injection profile of a well by sealing unwanted production (e.g., perforations) or thief zones, or repair a poor primary cement job before well completion. Squeeze cementing coupled with coiled tubing has been a standard remediation technique for shutting of unwanted gas or water production. Cement is able to fill perforation tunnels, channels behind pipe, and/or washout zones behind pipe, and consequently cement is able to provide a near wellbore block to production. Production from selected zones can then be reestablished by reperforating these zones. Unfortunately, cement has limitations as it does not penetrate into the porous rock, but rather fills and plugs voids, perforations, etc. as noted previously. As a result, microchannels along the cement and porous rock interface often develop due to cyclical changes in underground pressures and temperatures during subsequent production and shut-in stages.
Polymer gels or gel systems are also used for shutting of unwanted gas or water production and can be placed by bullheading or can be selectively placed through coiled tubing. The main difference from squeeze cementing is that the polymer gels provide in depth blockage of the formation by penetrating the porous media and crosslinking in situ therein. The in situ properties of these gels can be varied from flowing gels to ringing gels by adjusting the polymer concentration, the polymer molecular weight, and/or the type of crosslinker. A limitation of gels is that they may not have the mechanical properties to provide sufficient resistance to flow in the absence of a porous medium, for example in areas such as voids and cavities behind pipe.
A logical solution to the limitations outlined above is to combine polymer gels with cement squeezes to effectively block production through the porous medium, perforations, voids and/or cavities. This combination is typically conducted sequentially: first the polymer gel is placed in the formation and the treatment is completed with a cement tail-in to squeeze the perforations and any voids and cavities behind pipe. A disadvantage of the sequential combination treatment may be that the depth of polymer invasion in the porous media extends beyond the depths that can be penetrated by perforating guns and consequently the shut-off may be permanent.
Another approach to combining squeeze cementing and polymer gel technology for shutting of unwanted gas or water production is to use the polymer gel as the “mix water” for the cement slurry. The limited and controlled leak off of the polymer gel into the porous medium during the squeeze enables a controlled depth of invasion. U.S. Published Application No. 2003/0224946 A1, incorporated herein by reference in its entirety, discloses compositions that can be used for this combined gel-cement technique. One composition includes a crosslinkable material, e.g., H2ZERO polymer sold by Halliburton Energy Services of Duncan, Okla., for improving the strength of the composition when it sets such that it can withstand the pressures exerted by fluids in the subterranean formation. However, due to the alkalinity of the cement, which typically has a pH greater than 12, the gel time of the cement composition at the relatively high temperatures in the wellbore may be unacceptably short. The gel time refers to the period of time from initial mixing of the components in the cement composition to the point when a gel is formed. At this point, the viscosity of the cement composition is so high that it is no longer pumpable and thus does not reach the permeable zone where its placement is intended. A need therefore exists to control the gel time of such squeeze sealant compositions, thus ensuring that they can be properly placed in permeable zones downhole to prevent fluids from flowing into the wellbore. Furthermore, an ongoing need exists for improved gel systems and combined gel-cement techniques that limit leak off into the surrounding formation.